The present invention relates generally to the operation of fossil fuel-fired steam generator furnaces, and more particularly, to an improved method of introducing a dry sulfur oxide absorbent material, typically an alkali material such as limestone or lime powder, into a steam generator furnace firing a sulfur-bearing fossil fuel so as to optimize the efficiency of sulfur-oxide absorption over a wide range of furnace operating conditions.
Fossil fuels, such as oil, coal, peat, and lignite, are commonly used in steam generating furnaces to produce hot combustion products which in turn are used to generate steam to power electric producing turbine generators. However, due to the presence of nitrogen and sulfur compounds in such fossil fuels, the combustion of these fuels in steam generating furnaces creates obnoxious gases such as nitrogen oxides and sulfur oxides which must be removed from the flue gas generated in the furnace prior to venting the flue gas to the atmosphere. The formation of nitrogen oxides in fossil fuel fired furnaces is typically controlled by properly distributing the admission of combustion air to the furnace so as to provide a zone of initial combustion at substoichiometric levels and a subsequent secondary zone above the primary combustion zone to provide for complete combustion within the furnace. The removal of sulfur oxides from the flue gas on the other hand has typically been accomplished by scrubbing the product flue gas at a point downstream of the furnace after the gases have been cooled. Unfortunately, the equipment necessary to remove sulfur oxides from flue gases by scrubbing requires a large capital outlay and continued maintenance expenses to provide reliable operation.
Accordingly, effort has been made to remove the sulfur oxides from the flue gas within the furnace chamber itself by injecting a dry sulfur absorbing particulate material into the furnace. As the sulfur oxides are formed during combustion within the furnace, the sulfur oxides will, to varying extent, be absorbed by the particulate sulfur absorbing material within the furnace and carried from the furnace chamber entrained in the product flue gas. The sulfated absorbent particles will be removed in a downstream particulate collecter, such as an electrostatic precipitator, which is typically supplied with the steam generator to remove fly ash and other particulates from the flue gas prior to venting to the atmosphere.
Typically, the sulfur absorbing particulate material is added directly into the combustion zone of the furnace to be in intimate contact with the fuel during the combustion process. For example, U.S. Pat. No. 4,426,939 discloses injecting the sulfur oxide absorbing material into the stream of powdered fuel being passed from the pulverizer to the burners of the furnace. U.S. Pat. No. 4,262,610 discloses mixing the sulfur oxide absorbent material with the coal to be burned in the furnace upstream of the pulverizer so that the coal and the sulfur oxide absorbent are pulverized together. U.S. Pat. No. 3,746,498 discloses injecting the sulfur oxide absorbing material directly into the furnace combustion zone into a region of fuel-rich combustion. U.S. Pat. No. 3,520,649 discloses injecting a sulfur absorbent particulate material entrained in air directly into the high temperature burning zone. In each of these methods wherein the sulfur oxide absorbent material is actually present in the high temperature combustion zone of the furnace, the sulfur oxide absorbent material is subjected to extremely high temperatures and therefore may be deadburned thereby resulting in the material becoming ineffective as the sulfur oxide absorbent.
One method of avoiding deadburning is to inject the sulfur oxide absorbent into the furnace at a location downstream from the combustion zone wherein the gases have cooled to a temperature wherein deadburning does not occur. One such method is disclosed in U.S. Pat. No. 3,851,042 wherein fully hydrated lime particles are injected into the upper region of a furnace chamber in a stream of pressurized steam at a location wherein the gas temperature is in the range of about 650 C. to about 1300 C. As disclosed therein, the fully hydrated lime entrained in the pressurized steam is injected through an opening in the sidewall of the furnace across the path of the flue gases leaving the furnace chamber. One problem associated with this method of injection of the sulfur oxide absorbent particulate material downstream of the high temperature combustion zone is the difficulty of providing intimate mixing of the sulfur oxide absorbent with the flue gases so as to facilitate sulfur oxide absorption. In systems wherein the sulfur oxide absorbent is directly injected into the high temperature combustion zone, the sulfur oxide absorbent particulate material is present in the immediate vicinity of the combusting fuel particles. However, when injected into the product flue gas downstream of the combustion zone, the sulfur oxide absorbent material is not in intimate contact with the combusting fuel particles and therefore is not in the immediate vicinty of the sulfur oxide molecule as it is formed.
Accordingly, it is an object of the present invention to provide a method of injecting a sulfur oxide absorbent particulate material into a furnace to absorb sulfur oxide formed therein in such a manner as to ensure rapid and thorough mixing of the injected sulfur oxide absorbent with the product flue gases.